Make sure you can even install one.

They are not allowed under some codes.

They catch debris and clog easily, or fail to close as desired from debris.

The first step is to determine how storm runoff is getting into the sanitary sewer system. With some extremely rare exceptions it is illegal to connect downspouts or other storm drains to a sanitary sewer line.

"With some extremely rare exceptions it is illegal to connect downspouts or other storm drains to a sanitary sewer line."

The problem is often in older communities that at one point allowed storm water to be discharged into the sanitary system.

In one house I used to own (built in the mid 1950s) the basement perimeter drains, including an outside door drain 4 feet below grade and under-slab drain lines had all been legally connected to the sanitary system at the time.

The restrictions arose when sewage treatment came into use, but the old connections are normally grandfathered.

To add to the historical perspective by brickeye, some older municipal systems combined transport of sewage and surface run-off water. When it was not raining, everything went to a rudimentary sewage treatment plant. When it started to rain when the flow got to a rate that overwhelmed the capacity of the plant, everything went directly into the river or lake. The theory was that the great dilution is the solution. Water pollution standards changed.

When a second system was built, it was built for surface water and street drainage went to that. The combined systems were retained for waste water and hooked to fancy treatment plants. Old individual properties were not required to reroute the run-off. They might still discharge surface run-off into the sewage treatment system if the connections were not revised during some renovation.